Slide wire resistance



Jne 28, 1938. T` R. HARRISON ET AL SLIDE WIRE RES I STANCE Original Filed June 23, 1931' 3 Sheets-Sheet 1 FIG "3- June 28, 1938- T. R. HARRISON ET AL f -2,122,370

SLIDE WIRE RES ISTANCE Original Filed June 23, 1931 3 Sheets-Sheet 2 FIG 4 INVNTORS THOMAS RHARRISON, ERNEST H. GRAUEL a J.ERNEST KESSLER ,Bf m/MM@ ATTO RN EY June 28, 1938- T..R. HARRISON ET A1. 2,122,370

SLIDE WIRE RES I STANCE Original Filed June 23, 1931 3 Sheets-Sheet 3 INVENTORS THOMAS R-HARRISON., ERNEST H.GRAUEL 8: J.ERNEST KESSLER Patented June 28, 1938 NITED IIES SLIDE WIRE RESISTANCEy Thomas R. Harrison, Wyncote, Ernest H. Grauel,

Conshohocken, and

John Ernest Kessler,

Philadelphia, Pa., assignors to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Original application June 23, 1931, Serial No. 546,290. Divided and this application April 26, 1934, Serial No. 722,466

l1 Claims.

'Ihe general object of the present invention is the provision of an improved form of slide wire construction primarily devised for use in and as a part of a potentiometric measuring and re- 5 cording instrument. More specific objects of the invention are to provide a form of slide wire construction affording a greater facility of manufacturing process, to provide an improved form of construction and operation of the slide wire 10 per se, to provide a greater degree of convenience and efficiency of operation of the slide wire in conjunction with the related units of a potentiometer instrument, and to provide a slide wire assembly in the form of a separate slide wire assembly unit complete in itself for mounting in a frame in which other related parts, and in particular other related parts of a potentiometer instrument, may be mounted, so as to facilitate the manufacture, assembly and service of the slide wire assembly unit as well as of the related parts mounted in said frame. Other objects and advantages of the invention appear in the detailed description which follows:

in the accompanying drawings.

Fig. l is a cross sectional view of a slide wire unit and associatedparts showing the assembled relation of the unit to the frame and standard cell of a potentiometer instrument;

Fig. 2 is a detail showing, partly in section, of a portion of the slide wire assembly;

Fig. 3 is a view in perspective showing electrical connections for the slide wire unit;

Fig. 4 is a top plan view, partly broken away and in section, of the slide wire unit;

Fig. 5 is a view in iront elevation of the slide wire assembly unit; and

Fig. 6 is a diagrammatic view of the circuit of a potentiometer instrument .in which the slide wire assembly unit may be used.

4 The slide wire assembly unit shown in Figs. 1 to 5 was primarily devised for use in and as a part of a self balancing potentiometer measuring and recording instrument, such as is fully disclosed in our prior application, Serial No. 546,290, filed 45 June 23, 1931, of which this application is a division. One use of the instrument disclosed and claimed is illustrated diagrammatically in Fig. 6 of the drawings herein, which shows electric circuit arrangements of such a potentiometer instrument as is disclosed in our said prior application.

In Fig. 6, l designates a substantially constant source of electromotive force such as an 01- dinary dry cell connected in series with an adjustable resistor 253, the adjustment of which is adapted to vary vthe current flow in the circuit containing the cell I, a ballast resistor-3, a slide wire 245, another ballast resistor 5, and a calibrating resistor 6 which completes the circuit and the value and manner of connection of which may be varied in order to vary the measuring range of the potentiometer circuit. A shunt circuit containing resistors 1 and 8 is provided. The resistor 1 is included in the measuring circuit completed through resistor 9, galvanometer I0, the switch contacts II and I2 of a switch 210, conductors I3 and I4, across which are connected a source of potential, such as one or another of a series of thermocouples I5, the value of which is to be measured, conductor I6, resistor 240 and sliding contact 245 to the slide wire 240. By making the resistor 1 with a suitable temperature coefficient of electrical resistance, the variation in temperature of a thermocouple cold junction may be automatically compensated for, as is well known in the art. The resistor 8 is included in an auxiliary circuit extending from resistor 3 through resistor I9, standard cell 50, switch contacts II and 2I of switch 210, galvanometer il), and resistor 9, thus completing the circuit. A resistor 22 may be shunted about the galvanometer l in order to properly damp its movement by arranging the switch 210 to bring contact Il into engagement with the contact 22 at the same time that the contact II is brought into engagement with the contact 2 I.

In operation of the apparatus shown diagrammatically in Fig. 6, the source of electromotive force l supplies a steady current through slide wire resistor 240, thereby creating a denite potential drop across the resistor. The circuit including a thermocouple I applies an electromotive force on the slide wire 240 in a direction such as to oppose that created by the cell I. By adjusting the sliding contact 245 along the resistor 240, a point on the resistor 240 is reached where the potential of the thermocouple applied to the resistor 240 is equal and opposite to the potential of the resistor 240 at that point. At such time the galvanometer I0, which is normally ,connected in circuit with the thermocouple I5 by the switch 210, will have a zero deilection. Variations in the value, of the electromotive force of the thermocouple cause deection of the galvanometer I0 in a direction depending upon whether the variation is an increase or a decrease from that value for which theV potentiometer systemis previously balanced. The contact 245 may be adjusted again to bring the galvanometer into a position which corresponds to a balanced condition of the measuring system. When the potentiometer circuit is unbalanced a current, due to the thermocouple, ows through the circuit including the resistor 24U, and corresponding changes in the position of the contact 245 to balance the circuit, thereby varies the amount of electrical resistance due to resistor 240 which is included in the thermocouple and galvanometer circuit tending to change the constant of the galvanometer circuit. The resistor 24U is connected in series in the thermocouple and galvanometer circuit in such a manner that additional resistance is added from resistor 240 when a like amount of resistance is eliminated from resistor 24|) by movement of contact 245, thereby maintaining a constant electrical resistance in the thermocouple and galvanometer circuit so as to maintain an unvarylng galvanometer constant throughout the measuring range of the potentiometer.

Inasmuch as the circuit energizing cell I tends to deteriorate slightly with age and use, it'is necessary from time to time to adjust the resistor 253 in order to maintain a predetermined current flow through the slide Wire resistor 24U. For standardizing the current flowing in the slide Wire resistor, switch 210 is adjusted to bring the contact II into engagement with contacts' 2i and 22, thereby connecting the galvanometer in a circuit which includes the standard cell 5D as a source of electromotive force arranged to oppose the potential drop across the resistor 8 due to the current flowing in the resistor 8 from the ,cell i. Variations from the standard in the current flowing through resistor 8 from the cell I, cause a deflection of the galvanometer I0 from the position of zero deflection. The resistor 253 is then adjusted to bring the galvanometer to a position of zero deflection which corresponds to a balance between the potential drop across resistor 9 and the potential of standard cell 50 thereby standardizing the current.

Except for the galvanometer IU, the resistors 9 and I9 ,directly associated with the galvanometer, the standard cell 5U, the thermocouples I5 and means for connecting the latter to the switch contact I2 and resistor 240', all of the parts illustrated diagrammatically in Fig. 6 may advantageously be included in the slide wire assembly unit shown in Figs. 1-5, which, as shown, includes a gear 221 which is rotated to make slide wire adjustments as hereinafter described. In the self-balancing potentiometer instrument of our said prior application, the gear 221 is intermittently rotated in one direction or the other by relay mechanism controlled by the galvanometer I0, but that mechanism is not illustrated herein, as the present invention is not directly concerned with the means for, or matter of, eifecting the rotative movements of the gear wheel 221.

In the form of the slide wire' unit shown in Figs. 1-5, the gear 221 has a collar 228 carrying set screws for rigidly securing the gear 221 onto a shaft 229. The shaft 229 is Journalled in a hollow shaft 230 to which is secured by a set screw. or in any other suitable manner, the hub 23| of a serrated wheel 232. Hollow shaft 230 is journalled in collars or bushings 233 in a stationary tube. 234. A collar 235 is secured to the shaft 229 by a set screw and is interposed between gear 221 and wheel 232 so as to maintain shaft 229 in place when gear 221 is removed. The tube 234 is secured as by molding, or in any other suitable manner. into the molded support 238. Molding 239 has a top portion 231' adapted to form a cap for the slide wire unit and a cylindrical body portion 239 depending from the top portion 231. The cylindrical portion 238 is hollow and the members 229, 230, and 234 extend axially therethrough. The cylindrical portion 238 is formed, by molding. with a pair of helical grooves 239, which extend from top to bottom of the cylindrical portion, and which are adapted to receive the coiled electrical resistors 24U and 240. The electrical resistors 240 and 240' are each wound upon a corresponding body member of wire 24I, the ends of which are clamped beneath plates 242 by pressure exerted, the screws 243 securing the plates 242 to the molding into which the screws are threaded. The coiled resistors 24U and 240 when tightly arranged in the helical grooves 239, and the ends thereof secured by clamps 242, are adequately supported by the cylindrical body 236 of the molding.

The resistors 24D and 24U are evenly spaced throughout their length to provide a track 244 in which the slide wire contact 245 may ride. The contact 245 is a small cylindrical body of electrical conducting material of a size to span the track 244 formed between the resistors 240 and 24D and complete an electrical circuit therebetween, the resistors being bare along the points of contact. Roller contact 245 is arranged to ride in a slot 246 in a frame or plate 241, the roller contact being provided with reduced projections or pins 248, which engage the inner faces of the plate 241 adjacent the slot 246 and prevent the roller contact 245 from passing through the slot 246. The slot 246 extends transversely with re spect to the track 244,'and the plate 241 ls urged by spring 249 disposed about the axle for the plate and cooperating with a supporting frame 250 and the plate 241, so that the contact 245 is maintained in position between the slot 246 and the track 244 as the contact moves over the resistors. The track 244 provides a guide for moving the contact throughout the length of the resistors. The supporting frame 250 is molded into and supported by the arm 25| having a bushing 252 molded thereinto, in which the end of shaft 229 is secured by a force t, or in any other suitable manner. Movement of the shaft 229 by gear. 221 sweeps the supporting frame 253 over the surface of the helically arranged resistors 240 and 240', carrying the contact 245 along the track formed between the resistors as to thereby adjust the amount of electrical resistance in the measuring circuit of such a potentiometer as is illustrated in Fig. 6.

At the bottom of the cylindrical body of the A molding 239 is formed a pair of grooves adapted to accommodate a resistor 253 of the same type and mounted in the same manner as each of the resistors 240 and 240. The resistor 253 is formed in two parallel portions providing a track therebetween in which a contact 254, similar to contact 245, rides, being supported thereagainst by a plate 255, having a slot therein to accommodate the contact 254. Plate 255 is carried upon an arm 256 rigidly connected to the hollow shaft 230. Rotation of the serrated wheel 232 sweeps the plate 255 over the resistors 253, carrying the contact 254 along the track therebetween to adjust the electric resistance thereof. Adjustment of the contact 254 may be eilected by the manual rotation of wheel 232 to thereby standardize the current flowing through the potentiometer circuit.

The electrical connections for the resistors 243 and 240 and 253 are made by conductors 251 passing through apertures formed in the wall of the cylindrical molding 238 and united to the resistors by soldering or in any other suitable manzontally arranged conducting rods 258 are embedded in the body of a molding 260 adapted to be secured to molding 236 by screws 26|, or in any other suitable manner. When the molding 260 is secured upon molding 236 the conductors 259 make connection with the terminals oi conductors 258, the two being united by screws or soldering, or in any other suitable manner.

As shown more clearly in Fig. 3, some of the rods 259 are upturned at their other extremities. as indicated at 262, to form supports and electrical connections for spools 263 of electrical resistance wires which may constitute such potentiometer circuit resistances as those designated 3, 5, 6, 1 and 8, shown diagrammatically in Fig. 6. Suitable electrical conductors 264A are provided for joining the resistance spools electrically. Other of the conducting rods 259 extend downwardly and are provided with connecting clips 264, as shown in Figs. 1 and 3. The molding 236 is provided with apertures 265 to accommodate bolts 296 passing therethrough into threaded engagement with a supporting frame 41 thereby mounting the slide wire unit upon saidirame; In the use of the invention in our said prior application, the ira-me 41 is a potentiometer instrument frame member. Separately attached to the frame 41 is a molding 261 in which are embedded conductors 268, so arranged that when the molding 260 is secured in place upon the molding 236 and attached to the frame 41, the projections 264 of. conducting rods 259 are juxtaposed to the conductors 268, as shown in Fig. 1 where the conductors 264 and 268 may be united electrically by means of screws or in any other manner. Conductors extending into connection with a thermocouple I5 may be connected to the conducting bars 268 to place said thermocouple, the slide wire, resistors 240 and 240', and the calibrating resistance 253 in the electrical measuring circuit.

The previously mentioned switch 210 may be and preferably is included in the slide wire assembly unit. In the form of the unit shown in Figs. 1-5, the switch operations to eilect engagement of the switch contacts I I and I2, and the engagement of the switch contacts II with the contacts 2| and I2, as shown in Fig. 6, result from movements of a pivoted switch blade 212 relative to contacts 21|. The 'contacts 21| are formed as illustrated most clearly in Fig. 3 by bending up a portion of each of certain of conductors 259 so that the bent up portions are exposed at the surface of the molding 268. The switch blade 212 is of the spring brush type adapted to press against the contacts 21| and is carried by a shaft 213 ,having a flat thereon adapted -to prevent rotation of parts mounted thereon relative thereto. The switch blade 212 fits over the shaft 213 and is secured by bolt and nut cap 215, being secured thereto by the bolt 214.

Cap 215 is arranged to cover the switch contacts 21| when it is secured in position upon the molding 260 by screws 216. A rider ilts over the switch blade brushes and presses against the bottom surface of the top wall oi' the molding 215 to hold the brushes in wiping engagement with the contacts 21|. material of the cover 215 to limit the stroke of the switch, the stops cooperating with aV handle 219 of the switch. v

Inasmuch as in the use of the apparatus shown in Fig. 6, the calibrating resistor 240 should not be adjusted at any time other than when the switch is in such position as to connect the cali- Stops 218 are formed in the bratingresistor and standard cell .50 in series with the galvanometer IIJ, it is desirable to lock the wheel 232 against movement when switch 210 is in the position placing the galvanometer I9 in the measuring circuit. A spring 280, arranged along the wall of the molded cap 215, thrusts the switch blade 212 to the left, as shown in Fig. 4, in which position the galvanometer is placed in the measuring circuit. The operating handle 219 of the switch is provided with a projection 28| arranged to engage with the serrated periphery of the wheel 232 when the switch is in the position illustrated in Fig. 4. When it is desired to standardize the current in the potentiometer circuit, the operating handle 219 is manipulated manually to move the switch blade in a clockwise direction, as viewed in Fig. 4, thereby placing the galvanometer in series with the calibrating` resistor and standard cell, and at the same time moving the projection 28| out of engagement with the wheel 232 so that the adjustment of the calibrating resistor 253 may be effected manually. When the current has been calibrated, the switch handle 219 is released and the spring 280 moves the switch blade to that position placing the galvanometer in the measuring circuit and moving the projection 28| into locking engagement with the wheels 232.

Inasmuch as the resistance of the slide wire resistor, which is placed in circuit at the time that the potentiometer circuit is in balance,

provides the measure of the electromotive force under measurement, it is highly desirable to protect the slide wire resistor, and for similar reasons to protect the calibrating resistor from mechanical injury andv accumulation of foreign material which would tend to change the resistance thereof, or to cause deterioration thereof. vIt is also desirable, in order to attain great sensitivity, to provide a slide wire resistor of great length, and accordingly, by arranging the slide wire resistor asa helix upon a cylinder, as illustrated in Figs. 1 and 4, it becomes possible to employ a slide wire of great length arranged in such a compact manner as to aiord opportunity for enclosing the slide wire resistor and immersing it in a bath of insulating oil to protect the same. 'I'he arrangement of the cylindrical molding 238 in a vertical position, when the slide wire unit is arranged in the frame of the instrument, allows the use of a cup-like vessel 282 arranged to fit a rim 283 formed in the material of the molded cap 231, from which the cylindrical molding 238 depends. Recesses 284 (see Figs. 1 and 4) are formed in the molded cap 231, to accommodate the turned in ends 285 of a wire bail 286, which extends beneaththe vessel 282. A snap acting clamp 281 is arranged on the bail 286 to engage the bottom of the vessel 282 and force the same into sealing engagement with the rim 283 on the top of the molding 231, as shown in Fig. 1. By manipulation of the clamp 281, the bail 286 may be freed so that it may be swung out of the way and the vessel 282 removed. The vessel 282 may and preferably does contain oil rising to a height sufiicient to immerse the resistors on the cylindrical projection 238. A plug 288 is threaded into the cap 231 and may be removed to add oil to the vessel 282.

The molded cap 231 is provided with a projecvtion 289 (see Figs. l and 5) arranged to form a tend to ride up over the clamps 242, the projection 288 supports the plate 241 against such an outward thrust, thereby preventing the contact 245 from riding free of the track 240 at the ends thereof. The molding is provided with a lug 290 (see'Fig. l) which, in cooperation with frame 41, of the instrument, provides a bearing for the shaft i 14, forming a part of the mechanism disclosed in our said prior application not immedi ately related, except as noted, to the apparatus disclosed and claimed herein.

That portion of the frame 41, on which the slide wire unit is mounted, is provided with a recess 25! into which the standard cell 50 fits. The projection 261 on the molding 236, carries a clip 293 arranged to press against the standard cell and hold it in place.

The slide wire unit described hereinabove and embodying the features of this invention, is

formed largely from molded parts which may be constructed with a minimum of labor and material, and which, when formed, require no further fabricating operations thereon, and wherever possible other parts have been supported in said molded parts by embedding therein, and by employing such moldings the number of parts have been reduced to a minimum thereby sim plii'ying the operation of the assembled parts of the unit, and by arranging the support for the resistors as a vertical cylinder the use of an lmmersed slide wire of a great length is made possible. Furthermore the electrical and mechanical parts which are operatively associated in the potentiometer circuit are assembled for convenience-of operation in the unit, providing an exceedingly compact construction, reliable in operation, readily accessible for inspection and repair, and convenient for mounting in operative association with the other parts of the potentiometer instrument. Novel details of construction of the parts of the unit, and their manner of assembly, afl'ord efllciency and accuracy, and convenience of operation of the unit. The slide wire unit may be assembled on an instrument frame in a position where its condition and operation may readily be observed, and where the switch 210 is con veniently available for operation from the front of the instrument, and where the standardizing resistor 240 may be conveniently manipulated from the front of the instrument. The slide wire unit, the resistors associated therewith, and the standard cell, are arranged as shown in a convenient, compact group readily accessible for making the electrical connections thereto, and conveniently arranged for servicing and the slide wire unit is demountable from its supporting frame as a unit, thereby facilitating' replacement of parts.

While in accordance with the provisions of the statutes, we have illustrated and describedthebest form of embodiment of ourinvention nw known to us, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of our invention as set forth in the appended claims and that in some cases certain features of our invention may be used to advantage without a corresponding use of other features.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

1. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire wound about said support and comprising side by side convolutions at opposite sides of a circumferentially extending strip portion of said surface, a cylindrical Contact member having transverse end surfaces and having its axis transverse to the first mentioned axis and having its periphery in engagement with the convolutions at the side of said strip, and a guide overhanging said convolutions and revoluble about the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in guiding relation with said transverse end surfaces.

2. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire wound about said support and comprising side by side convolutions at opposite sides of a circumferentially extending strip por'- tion of said surface, a cylindrical Contact member having transverse end surfaces and having its axis transverse to the rst mentioned axis and having its periphery in engagement with the convolutions at the side of said strip, and a guide overhanging said convolutions and revoluble about the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in guiding relation with said transverse end surfaces, said member having axial extensions from said transverse end sur- .faces engaging the side of said guide adjacent said surface at the opposite sides of said slot.

3. In combination, a support having a surface extending cylindrically about` an axis, electric resistance wire wound about said support and comprising side by side convolutions at opposite sides of a circumferentially extending strip portion of said surface, a cylindrical contact member having transverse end surfaces and having its axis transverse to the first mentioned axis and having its periphery in engagement with the convolutions at the side of said strip, a guide overhanging said convolutions and revoluble about the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in guiding relation with said transverse end surfaces, said member having axial extensions from said transverse end surfaces engaging the side of said guide adjacent said surface at the opposite sides of said slot, and means resiliently pressing said guide member toward the first mentioned axis.

4. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire wound about said support and comprising side by side helical convolutions at opposite sides of a circumferentially extending strip portion of said surface, forming a contact track, a cylindrical contact member having transverse end surfaces and having its axis transverse to the axis of said surface and having its periphery in engagement with the convolutions at the side of said track, an arm revoluble about the first mentioned axis, a guide hinge-connected to said arm to turn relatively to the latter about an axis parallel to the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in guiding relation with said transverse end surfaces, said member having ax ial extensions from said transverse end surfaces engaging the side of said guide adjacent said surface at the opposite sides of said slot, and means acting between said arm and guide resiliently pressing the latter toward the first menresistance wire helically Wound about said support and comprising side by side helical convolutions at opposite sides of a helically extending strip portion of said surface forming a contact track, a cylindrical contact member having transverse end surfaces and having its axis transverse to the axis of said surface and having its periphery askew to, and in engagement with the convolutions at the side of said track, and a guide overhanging said convolutions and revoluble about the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the rst mentioned axis and in guiding relation with said transverse end surfaces.

6. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire helically wound about said support and comprising side by side helical con@ volutions at opposite sides of a helically extending strip portion of said surface forming a contact track, a cylindrical contact member having transverse end surfaces and having its axis transverse to the axis of said surface and having its periphery askew to, and in engagement with the convolutions at the side of said track, and a guide overhanging said convolutions and revoluble about the iirst mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in ,guiding relation with said transverse end surfaces, said member having axial extensions from said transverse end surfaces engaging the side of said guide adjacent said surface at the opposite sides of said slot.

'7. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire helically wound about said support and comprising side by `side helical 'convolutions at opposite sides of a helically extending strip portion of said surface forming a contact track, a cylindrical contact member having transverse end surfaces and having its axis transverse to the axis of said surface and having its periphery askew to, and in engagement with the convolutions at the side of said track, a guide overhanging said convolutions and revoluble about the iirst mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the first mentioned axis and in guiding relation with said transverse end surfaces, said member having axial extensions from said transverse end surfaces engaging the side of said guide adjacent said surface at the opposite sides of said slot, and-means resiliently pressing said guide toward the first mentioned axis. y

8. In combination, a support having a surface extending cylindrically about an axis, electric resistance wire helically wound about said support and comprising side by side helical convolutions at opposite sides of ahelically extending strip portion of said surface forming a contact track, a cylindrical contact member having transverse end surfaces and having its axis transverse to the axis of said surface and having its periphery in engagement with the ccnvolutions at the side of said track, a guide overhanging said convolutions and revoluble about the first mentioned axis and formed with a slot into which said member extends and having opposite side edges parallel to the rst mentioned axis and in guiding rela- -tion with said transverse end surfaces, and means through which said guide presses said member against said track and the adjacent convolutions.

9. In combination, a cylindrical support, electric resistance Wire wound helically on said sup-h port to provide side by side spaced apart convolun tions, said resistance wire extending radially outward from the peripheral surface of said support,` a member adapted to be rotated about the axis of said support and comprising a portion adjacent but spaced outwardly from said surface and formed with a guide parallel to said axis, and a bridging contact of conducting material and extending into the space between adj accnt portions of side by side convolutions and being in wedging engagement with said portions and therebyheld out of engagement with said surface, said member being adapted to resiliently press said contact inward toward said axis, whereby said wedging engagement is maintained and said contact is moved helically about said axis when said member is rotated about said axis.

10. In combination, a first electrical resistance adapted to form a slide wire for a potentiometer circuit, an auxiliary electrical resistance adapted to form the calibrating resistance for the potentiometer circuit, electrical contacts for each of said resistances, first means 'for relatively adjusting the first resistance and its contact, second means for relatively adjusting said auxiliary resistance and its contact, and a switch adapted to change the potentiometer circuit from a measuring circuit to a calibrating circuitsaid switch including an arm adapted to engage second mentioned means and prevent operation of the latter when said switch is in position to energize the measuring circuit, and to disengage said second mentioned means and permit operation of the latter when said switch is in position to energize the calibrating circuit, each of said means comprising a shaft, one of said shafts being tubular and coaxial with'and surrounding the other, and said arm being movable angularly about an axis laterally displaced from the common axis of said shafts into and out of engagement with said second mentioned means.

11. In combination, a first electrical resistance adapted to form a slide wire for a potentiometer circuit, an auxiliary electrical resistance adapted to form the calibrating resistance for the potentiometer circuit, electrical contacts for each of said resistances, first means for relatively adjusting the rst resistance and `its contact, second means for relatively adjusting said auxiliary resistance and its contact, and a switch device adapted for adjustment to change the potentiometer circuit from a measuring circuit to a calibrating circuit, and including locking means adapted to engage the second mentioned means and prevent operation of the latter when said switch device is adjusted to energize the measuring circuit, and to disengage said second men tioned means and permit operation of the latter when said switch device is adjusted to energize the calibrating circuit, each of said means comprising a shaft, one of said shafts being tubular and coaxial with and surrounding the other, and said locking means being movable angularly about an axis laterally displaced from the common axis y of said shafts into and out of engagement with 

